The present invention relates to an image reversal electrostatographic apparatus for producing positive electrostatic copies of negative microfilm frames or the like.
In the conventional electrostatic process a photoconductive drum, belt or the like is uniformly charged and radiated with a light image of an original document such as a printed rage. The light image causes localized photoconduction and the formation of an electrostatic image on the drum. A toner substance is applied to the drum to develop the electrostatic image into a toner image which is transferred and fixed to a copy sheet to provide a permanent reproduction of the original document.
This process is a non-image reversal process, producing a positive copy of a positive document (or a negative copy of a negative document), and is not suitable for producing a positive copy of a negative microfilm frame or the like.
A problem inherent in the standard process is that it is impractical to use the entire surface of the photoconductive drum for electrostatic image formation. Therefore, non-image or border areas are provided around the image area. If the entire surface of the drum is electrostatically charged prior to imaging, the electrostatic charge in the non-image areas will be maximum and a maximum amount of toner substance will adhere thereto. This toner must be removed from the drum by a cleaning unit, often overloading the cleaning unit, and constitutes a major source of waste.
In an attempt to eliminate these undesired effects, it has been practiced in the prior art to eliminate charging the non-image areas by means of a timer or the like connected to control the charging means. In other words, the preliminary electrostatic charge is applied only to the image area of the drum.
In order to provide positive copies of negative original documents such as microfilm frames, a modification of the standard process has been developed. Whereas in the standard process toner particles charged with a polarity opposite to that of the electrostatic image are used, in the image reversal process toner particles charged with the same polarity as the electrostatic image are used. These particles adhere to the areas of low, rather than high electrostatic charge, and produce a reversed image copy.
However, where the non-image areas of the drum are not charged, the electrostatic potential in such areas being zero, a maximum amount of toner adheres to these areas in the image reversal process. This toner must be removed as in the standard process and constitutes a major source of waste as well as overloading the cleaning unit.